Transmission - it seems simple (and cheap) to use the 5 speed
transmission,
with driveshaft attached to prop shaft with CV-joint and thrust bearing.
This would allow some variation in reduction ratio, for varying
conditions.
Am I nuts?
///
Thanks - Sal's Dad


In the matter of a variable gear transmission: yes, probably.

Brian W

I've done some preliminary calculations, using Gerr's "Propeller Handbook"
as a reference. Based on two boats of similar hull design (I'm sure there
are more out there, but I haven't located them) and Atkins' estimates, I ran
numbers for very light (2,000 lbs hull and machinery, minimal passenger and
fuel, no cargo) medium (1,000 lbs load) and heavy (2.000 lbs). The engine
claims 55 hp @4,000 rpm, so I've assumed 40 hp @3200 rpm cruising.

For medium load, 40 hp should deliver up to about 16 knots, 55 hp about 19.
With a 1.4:1 reduction (3rd gear), according to Gerr's formulas, a 10 pitch
prop will deliver 15 knots, 19 @4,000 rpm. 12" diameter is ideal

Lightly loaded, 40hp should provide 20 kts, 55 hp 26kts. With a 1:1
reduction (4th) a 10 pitch prop would push it at 21/26.5. 10" diameter
would be adequate.

Heavily loaded, you could expect 14 kts with 40hp, 15 or so with 55 hp. A
2.1:1 reduction would be pretty low, 10.6/12.5 knts, and it could turn a 16"
prop (can you say "tugboat"?). In any case, there's no room for a prop
bigger than 12"; it is tucked up above the box keel.

Seems to me that varying reduction ratios is a reasonable way to improve
performance with widely varying loads - 50% of the time would be "normal"
operation (with the prop optimized), 25% carrying cargo, a big crew, towing,
or heavy weather - and 25% would be just the skipper, out joy-riding in flat
water.

I don't see how this use would do extraordinary damage to the tranny - it
would only be operating a few hundred hours a year. And the separate thrust
bearing and u-joint would keep it isolated from the prop shaft.